A new medical syndrome, Acquired Immune Deficiency Syndrome (AIDS) has recently been recognized. It is characterized by profound progressive depression of the immune system, resulting in repeated opportunistic infections and at least one type of neoplasm. The syndrome is usually if not always fatal. It is epidemic in the United States among certain "high-risk populations (male homosexuals, illicit drug users, Haitians, and, to a lesser extent, hemophiliacs, recipients of blood transfusions and intimate contacts of cases). The epidemiology of the syndrome suggests that it is caused by a transmissible agent. Attempts to transmit a putative AIDS agent to chimpanzees and monkeys are in progress.